Method of coating and bonding refractory-base-metal articles



g- 2, 1966 H. w. JACOBSON 3,263,325

METHOD OF COATING AND BONDING REFRACTORYBASE-METAL ARTICLES Filed May 7, 1965 INVENT OR HOWARD W. JACOBSON ATTORNEY United States Patent 3,263,325 METHOD OF COATING AND BONDING REFRAC- TORY-BASE-METAL ARTICLES Howard W. Jacobson, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed May 7, 1963, Ser. No. 278,706 3 Claims. (Cl. 29492) This invention relates to processes for improving the high temperature oxidation resistance of refractory metals and their alloys by coating them with alloys of titanium or chromium, and is more particularly directed to such processes for coating refractory-base-metal articles, said metal being selected from the group consisting of columbium, columbium-base alloys in which the columbium content is at least 50% by weight, tantalum, and tantalum-base alloys in which the tantalum content is at least 50% by weight, said processes comprising the step of heating the refractory-base-metal article for a period of from 2 to 5 hours, at a temperature of from 800 C. to 1000 C., in a molten salt bath comprising (1) a salt selected from the group consisting of alkali and alkaline earth metal halides, (2) at least one metal chloride selected from the group consisting of titanium dichloride and chromium dichloride, and (3) at least two metals selected from the group consisting of titanium, chromium, aluminum, vanadium, and zirconium, at least one of these metals being identical with a metal whose chloride is chosen from group (2), whereby an alloy coating of the metals selected from group (3) is formed on the surface of said refractory metal article.

In the figure of the drawing, there is shown in cross section the application of a suitable apparatus to a coating process of this invention, the process being carried out in a fused (that is, molten) salt bath.

In recent years there has been an increase in demand for refractory metals or refractory metal alloys able to withstand high stresses at elevated temperatures and also able to withstand exposure to oxidizing gases orother corrosive materials at elevated temperatures. Columbium and tantalum have been found to be refractory metals highly suited to many applications where high strength metal is needed. Particular advantage has been found for the metal columbium in that it is a relatively light weight metal and has a low neutron-capture cross-section.

A major disadvantage in the use of columbium and tantalum has been that they are susceptible to oxidation, particularly at higher temperatures. Much effort has been expended to increase the oxidation-resistance of 'these metals by the addition of alloying elements, and some alloys have been prepared which exhibit quite a high degree of oxidation resistance. It has been found that, unfortunately, when oxidation-resistant alloys of columbium and of tantalum are prepared, very often the valuable properties of high temperature strength and fabricability have been minimized. Conversely, where alloys have been fabricated with the object of increasing their high temperature strength, the oxidation resistance of these alloys has been unacceptable. Thus, there is an increasing interest in coating these refractory metals and their alloys to improve their oxidation resistance, while retaining the advantages of high temperature strength of the underlying metal or alloy.

portant that the coating process be one in which various shapes and sizes of articles can be conveniently treatedl It will be advantageous also, if the process be one in which the articles can be coated within a relatively short time.

3,263,325 Patented August 2, 1966 The coatings which are applied to the refractory metals and alloys by the processes of this invention can be of chosen compositions to serve as brazing alloys. Also, alloyed compositions which may be applied to refractory metals and alloys by such processes may be chosen to permit subsequent diffusion bonding.

It is therefore an object of this invention to prepare coated articles in which there has been formed upon an underlying refractory-metal base an adherent alloy layer comprising at least one of the metals titanium and chromium. These alloy coatings are formed on the surface of the refractory-metal base by means of a fused salt bath process. Another object is to prepare coated refractory metal alloys which are able to withstand mechanical stresses at elevated temperatures over long periods of time. Another object is to provide processes for applying to refractory metals coatings which are very adherent and which resist spalling and cracking even under conditions involving thermal cycling at extremely high temperatures in oxidizing atmospheres and under conditions of thermal shock at high temperatures. A further object is to provide processes for applying coatings which impart a very high degree of resistance to oxidation to the refractory metal-base. Another object is to provide coating processes which permit retention of the fabricability and strength properties of the original, uncoated refractory metal or alloy. A further object is to provide processes for applying surface alloys which will serve as subsequent brazing and diffusion bonding composition for the articles so treated. Further objects will appear hereinafter.

The foregoing and related objects are accomplished according to this invention, and refractory-base-metal articles of columbium or tantalum or their alloys are provided with an alloy coating comprising titanium or chromium or both to enhance their properties in the desired manner, by processes which include the step of heating the articles for 1 to 5 hours at a temperature of 800 to 1000 C. in a molten salt bath comprising (1) a halide of an alkali metal or alkaline earth metal, (2) titanium dichloride of chromium dichloride and (3) at least two metals from the group including titanium, chromium, aluminum, vanadium, and zirconium, at least one of the selected metals being identical with a metal whose chloride was chosen in (2).

Processes employing fused salt baths for applying certain metal coatings to an underlying metal base are already known and have been practiced in the art. However, no method has been disclosed by which alloys comprising at least one of the metals titanium and chromium may be plated out to form a highly protective alloy coating on columbium, columbium-base alloys, tantalum or tantalum-base alloys. For the purposes of this invention columbium-base alloys and tantalum base alloys are to be considered alloys comprising the said refractory metals in the proportion of at least 50% by weight.

By the processes of this invention, alloys can be coated on unalloyed columbium or tantalum, or alloys may be coated on alloys of either of these refactory metals. Where alloys are being deposited onto unalloyed metal, the unalloyed metal combines with the alloys being deposited and forms a continuous, diffused layer therewith. Similarly, alloys deposited onto refactory-metaLbase alloys form therewith an intermediate layer comprising the metals present both in the coating alloy and in the underlying alloy-metal base.

The processes of this invention can advantageously be carried out in an apparatus as shown in the figure. This apparatus comprises a stainless steel reactor 1 having a mild steel liner. Specimens 2 to be coated are suspended by a tantalum wire 3, making it possible to raise or lower the specimens. A gas inlet 4 permits the introduction of 'an inert gas. An outlet 5 allows for the exhaust of gases from the reactor. The gas inlet can be submerged if agitation of the fused salt bath is desired. To afford rapid cooling of the specimens which have been coated, a necked portion at the top of the reactor is surrounded by cooling coils 6. The fused salt 7 contains the lower halides of the metals which are to be deposited on the specimens. Particles of metals 8, of which the lower halide of at least one is present in the salt bath, are present in the bottom of the reactor, serving to maintain reduction of the lower halides in the salt bath.

A process of the invention is carried out by preparing a fused salt bath comprising an alkali or alkaline earth metal salt bath and a lower halide of at least one of the metals titanium and chromium. The alkali or alkaline earth metal salt used should be one or a combination of salts which will be molten at a temperature of about 800 C. A preferred salt is sodium chloride. To this fused salt bath there is added a quantity of the lower halide of titanium, or of chromium, or of both. The lower halide or halides preferably is present in the proportion of from about 5 mole percent to about mole percent of the fused salt bath. The reduced condition of the lower halide salt is maintained by equilibrating the salt bath with powdered titanium, or powdered chromium, or with powdered Ti-Cr alloy. If the alloy coating desired is one comprising a metal or metals in addition to one or both of titanium and chromium, a quantity of the metal or metals desired is also added to the salt bath.

The process is initiated by heating the fused salt bath to a temperature in excess of 800 C. but below the boiling point of the fused salt bath, preferably to not more than 1000" C. A temperature from 900-950 C. is satisfactory for the coating of most refractory-metal-base alloys. Into the fused salt bath are placed the articles to be coated. The thickness of the coating desired is regu lated by the time and temperature'of immersion of the articles. The coating rate is approximately 0.5 mil/hour when a temperature of 950 C. is maintained and no means of agitation of the salt bath is provided. Although a desirable coating can be obtained on the metal articles by suspending them in the quiescent fused salt bath, it has been found that a faster rate of coating, and in some cases a more uniform coating, can be obtained if some form of agitation is provided. By bubbling argon through the molten bath, a rate of coating of from about 1.0 to about 1.5 mils/hr. can be effected. Coatings of the desired thickness are obtained under the above-described conditions by continuing the heating for a period of about from 1 to 5 hours.

Although the chemical nature of the reactions by which alloys are coated in the fused salt bath is not readily apparent, it is believed that the salts in the bath, and particularly the lower halides of titanium and chromium, act as transport agents in depositing on the refractory-metal article the metals comprising the alloy coatings. If, for example, the desired coating is to be one of Ti-Cr alloy, the lower halides in the salt bath are thought to disproportionate according to the following reactions:

Since excess titanium and excess chromium are both present in the salt bath, the two reactions become reversible and additional CrCl and TiCl become available for deposition of more titanium and chromium on the article to be coated. Where another metal, for example aluminum, is to be one of the components of the alloy coating, the reactions by which the metal is transported are not completely understood; however, it has been found that when the halide of at least one of the metals titanium and chromium is present in the fused salt bath, additional metals present in the elemental state will act with the titanium or chromium or both to form adherent alloy coatings.

Refractory metals which have been satisfactorily coated by the process of this invention include columbium metal; a columbium-base alloywith 1% zirconium; a columbium-base alloy with 10% titanium and 5% zirconium; and a columbium base allow with 10% titanium and 10% molybdenum; also, tantalum; and a tantalum-base alloy with 10% .by weight of tungsten. The composition of the refractory-metal base is not critical, and the processes of this invention can be applied to any columbiumor tantalum-base article or articles of their alloys comprising greater than 50% by weight of the refractory metal.

For a more specific description of the manner in which the processes of the invention may be carried out, the following illustrative examples are given.

Example 1 A salt charge weighing 3 lbs. and comprising 60 mole percent NaCl, 20 mole percent TiCl and 20 mole percent CrCl was placed in a stainless steel reactor having a mild steel liner, as shown in the figure. Into the reactor was introduced also 25 grams of powdered alloy of Ti-Cr which 'had been prepared by melting together the constituent elements in equal parts by weight and comminuting the cast alloy. The temperature of the salt was raised to 800 C. and the salt bath allowed to come into equilibrium with the titanium-chromium powder in the bot tom of the container, by holding at temperature for approximately 1 hour. An alloy coupon of D-ll, an alloy of columbium containing 1% zirconium, was lowered into the fused salt bath by means of a tantalum wire. The bath temperature was raised to 900 C. and maintained at this level for a period of 2 hours. During heating a flow of argon was maintained over the surface of the fused salt bath. After two hours the coupon was raised with the tantalum wire, and allowed to cool in the upper zone of the reactor. The coupon was then removed from the reactor, and washed free of salt with a 5% I-INO solution. Examination showed that a l-mil-thick coating had been deposited on the metal coupon. The coated coupon was studied in cross-section and an outside layer firmly bonded to the underlying metal was found to be plainly visible over the underlying base alloy. Spectrographic analysis of this coating showed it to be 65% titanium, 30% chromium, and 5% columbium.

Example 2 Using the process of Examples 1 and 2 above, coupons of other samples of D-11 columbium-base alloy (columbium containing 1% zirconium) were coated by immersing for two hours in a salt bath at a temperature of 900 in an inert atmosphere. The composition of the salt bath, the thickness of the coating, and composition of the coating were as follows: I

TABLE 1 Ex. No. Salt Composition Coating Coating Composition Thickness 3 TiOlg 18 mol percent; 1 mil wt. percent Ti-5 I Cllg, 2 mol per- Wt. percent (Jr-5 pent; Bal. NaCl. wt. percent Nb. 4 T101 20 mol percent; d0 60 wt. percent Ti-3O Bal. NaCl in equll. wt. percent (Jr-10 with Tl-Cr powder. wt. percent Nb.

Example 5 A salt bath of NaCl containing 20 mol percent TiCl was placed in equilibrium with 25 grams of chromiumaluminum alloy (50-50 wt. percent). The salt bath was heated to a temperature of 800 C. and coupons of alloy D-31 Ti, 10% Mo, bal. Nb), D36 (10% Ti, 5% Zr, bal. Nb), and alloy 13-11, were lowered into the bath. The temperature of the bath was raised to 900 C. while argon was bubbled through it to provide agitation. The samples were allowed to remain suspended in the bath for lhour. Coatings approximately 1 mil thick were deposited on each of the three alloy samples; these coating were found to contain aluminum, chromium, and traces of columbium.

Example 6 A salt bath of the same composition as that of Example 5 was heated to 800 C. and placed in equilibrium with a vanadium-chromium-aluminum alloy comprising equal parts by weight of these three metals. Specimens of alloys D-31, D-36, and 13-11 were lowered into the bath when it had attained this temperature. The bath temperature was then raised to 900 C. and maintained at this level for 1 hour. The alloy samples were removed from the bath, cooled to room temperature in an inert atmosphere, removed from the reactor, and washed in 5% HNO These were found to be coated to a thickness of about 1 mil. Spectographic analysis of the coating showed it to be titanium-vanadium-chromium-aluminum with traces of columbium.

Two of these specimens were joined by placing them in contact with each other at 1500 C. for one hour. It was found that the surface coating composition had melted and had formed a brazed joint.

Example 7 According to a process of Example 1, 4 coupon specimens of D31 alloy were coated to a depth of 0.5 mil with Ti-Cr alloy. In this case, argon was bubbled through the molten salt to agitate it, and the time for applying a coating 0.5 mil in thickness was 1 hour at a temperature of 850 C.

These specimens were cooled in the upper part of the reactor, then removed and washed in 5% HNO to remove excess salt.

The four samples thus coated with Ti-Cr were further treated to apply a layer of silicon over the Ti-Cr coating by means of a fluidized bed technique as follows.

The Ti-Cr coated specimens were suspended by a tantalum wire in the upper part of a quartz reactor 4" in diameter and 40 high having a bottom gas inlet and an upper outlet for exhaust gases. The inlet was fitted with a porous quartz plug. The reactor was charged to a level of about 2 inches with silicon of +40 mesh size. Over this silicon was charged very pure silica powder of 40+80 mesh size to a level of about 12 inches in the reactor. The reactor was connected to a bulb-type vaporizer in which about 450 grams of iodine was placed and through which vaporizer a flow of argon could be passed to carry iodine vapor into the reactor. Through the bottom inlet of the reactor argon was fed at a velocity of 0.2 to 0.3 ft./sec., whereby the bed of silicon and silica was fluidized. The reactor was heated to a temperature of 900 C., and the specimens were then lowered into the upper portion of the fluidized bed. The iodine vaporizer was heated to 120-130 C. and a flow of argon was started over the iodine to carry iodine vapor into the reactor. Heating was continued until the temperature in the fluidized bed had reached 1000 C., and the operation was continued under these conditions for 4 hours. The samples were then lifted to the upper portion of the reactor, the iodine-argon flow cut off, and the reactor cooled While the flow of argon to fluidize the bed was continued.

The samples were removed from the reactor and were found to have a silicon coating 1 mil in thickness overlaid on the Ti-Cr coating.

The four D31 coupons thus coated with 0.5 mil Ti-Cr and 1.0 mil Si were tested for resistance to oxidation by thermal cycling. This test consisted of maintaining the samples at a temperature of 1370 C. for one hour in flowing air and cooling to room temperature, this heating and cooling constituting one thermal cycle. Each of the 4 coated specimens of D-31 withstood 30 such thermal cycles before failure of the coatings. There was no spalling or cracking of the coating on any of the samples during the thermal cycling.

Example 8 A salt bath of the composition used in Example 5 was placed in equilibrium with a titanium-aluminum alloy containing 90 wt. percent Ti and 10 wt. percent Al. Specimens of alloys D-31, D-36, and D-ll were heated to 800 C. above the salt bath, and lowered into the salt bath when its temperature reached 800 C. The salt-bath temperature was raised to 900 C. and maintained at this level for 2 hours. An inert atmosphere was maintained in the reactor by a flow of argon over the bath surface. An adherent coating of approximately 1 mil thickness was deposited on the columbium-alloy coupons by this process. The coating on the coupon was found to be titaniumaluminum-columbiuin.

Example 9 Using the procedure of Example 1, a fused salt bath of mole percent NaCl 20 mole percent TiCl was placed in equilibrium with Ti-Cr alloy powder of 50-50 wt. percent at 900 C. Coupon specimens of pure tantalum sheet were lowered into the bath and held there for four hours. The specimens were then removed and the 1.2 mil coating so deposited was analyzed spectrographically and found to contain titanium, chromium, and tantalum.

Example 10 Using a salt bath of the composition used in Example 9, coupon specimens of a tantalum alloy containing wt. percent Ta and 10 wt. percent W were heated to 800 C. and the fused salt bath was heated to the same temperature. The specimens were lowered into the bath and the temperature was raised to 950 C. The specimens were kept in the bath at this temperature for four hours and were then removed, cooled and washed to remove the salt. The coupons were found to have a 1.4 mil coating consisting of Ti-Cr-Tawith traces of W.

It will be seen from the above disclosure that protective alloy coatings comprising the metals titanium, chromium, vanadium and aluminum have been placed on columbiumbase alloys by a method of this invention. It will be understood that other alloy coatings can be placed upon columbium-base alloys by this technique.

It is important to maintain the valence of the coating metal at its lower level in the salt bath if coating on columbium-base alloys is to be completely satisfactory. This low valence is maintained by having present in the salt bath elemental metal corresponding to the metal in the titanium or chromium sub-chlorides. For example, in cases where a Ti-Cr coating is desired, an alloy comprising Ti and Cr is placed in contact with the salt bath to maintain the Ti and Cr in the salt bath at a valence near 2.

Columbium-base articles coated according to a process of this invention have been found to be particularly resistant to corrosion or oxidation at elevated temperatures. This is shown by the cycling tests to which these coated speciments have been subjected. Also, the coated colum bium base alloys maintain high strength and their original fabricability because they have not been heated above the temperature at whichductile-brittle transition takes place.

I claim:

1. In a process for coating a refractory-base-metal article, said metal being selected from the group consisting of columbium, columbium-base alloys in which the columbium content is at least 50% by Weight, tantalum, and tantalum-base alloys in which the tantalum content is at least 50% by weight, the step comprising heating said refractory metal article for a period of about from 1 to hours, at a temperature of 800 C. to 1000 C., in a molten salt bath comprising (1) a salt selected from the group consisting of alkali and alkaline earth metal halides, (2) at least one metal chloride selected from the group consisting of titanium dichloride and chromium dichloride and (3) at least two metals selected from the group consisting of titanium, chromium, aluminum, vanadium, and zirconium, at least one of these metals being identical with a metal whose chloride is chosen from group (2), whereby an alloy coating of the metals selected from group (3) is formed on the surface of said refractory metal article.

2. In a process for bonding a plurality of refactorybase-metal articles, said metal being selected from the group consisting of columbium, columbium-base alloys in which the columbium content is at least 50% by weight, tantalum and tantalum-base alloys in which the tantalum content is at least 50% by weight, the steps comprising (a) heating said refractory metal article for a period of about from 1 to 5 hours, at a temperature of 800 C. to 1000 C., in a molten salt bath comprising 1) a salt selected from the group consisting of alkali and alkaline earth metal halides, (2) at least one metal chloride selected from the group consisting of titanium dichloride and chromium dichloride and (3) at least two metals selected from the group consisting of titanium, chromium, aluminum, vanadium, and zirconium, at least one of these metals being identical with a metal whose chloride is chosen from group (2), whereby an alloy coat ing of the metals selected from group (3) is formed on the surface of said refactory metal article, (b) placing a plurality of said coated articles in contact with each other, and (c) subjecting said contacted articles to an elevated temperature at least suflficiently high to effect diffusion bonding thereof.

3. In a process for bonding a plurality of refactorybase-metal articles, said metal being selected from the group consisting of columbium, columbium-base alloys in which the columbium content is at least by weight, tantalum and tantalum-base alloys in which the tantalum content is at least 50% by Weight, the steps comprising (a) heating said refactory metal article for a period of about from 1 to 5 hours, at a temperature of 800 C. to C., in a molten salt bath comprising (1) a salt selected from the group consisting of alkali and alkaline earth metal halides, (2) at least one metal chloride selected from the group consisting of titanium dichloride and chromium dichloride and (3) at least two metals selected from the group consisting of titanium, chromium, aluminum, vanadium, and zirconium, at least one of these metals being identical with a metal whose chloride is chosen from group (2), whereby an alloy coating of the metals selected from group (3) is formed on the surface of said refactory metal article, (b) placing a plurality of said coated articles in contact with each other, and (c) subjecting said contacted articles to an elevated temperature sufficiently high to effect brazing together thereof.

References Cited by the Examiner UNITED STATES PATENTS JOHN F. CAMPBELL, Primary Examiner. 

1. IN A PROCESS FOR COATING A REFRACTORY-BASE METAL ARTICLE, SAID METAL BEING SELECTED FROM THE GROUP CONSISTING OF COLUMBIUM, COLUMBIUM-BASE ALLOYS IN WHICH THE COLUMBIUM CONTENT IS AT LEAST 50% BY WEIGHT, TANTALUM, AND TANTANLUM-BASE ALLOYS IN WHICH THE TANTALUM CONTENT IS AT LEAST 50% BY WEIGHT, THE STEP COMPRISING HEATING SAID REFRACTORY METAL ARTICLE FOR A PERIOD OF ABOUT FROM 1 TO 5 HOURS, AT A TEMPERATURE OF 800*C. TO 1000*C., IN A MOLTEN SALT BATH COMPRISING (1) A SALT SELECTED FROM THE GROUP CONSISTING OF ALKALI AND ALKALINE EARTH METAL HALIDES, (2) AT LEAST ONE METAL CHLORIDE SELECTED FROM THE GROUP CONSISTING OF TITANIUM DICHLORIDE AND CHROMIUM DICHLORIDE AND (3) AT LEAST TWO METALS, SELECTED FROM THE GROUP CONSISTING OF TITANIUM, CHROMIUM, ALUMINUM, VANADIUM, AND ZIRCONIUM, AT LEAST ONE OF THESE METALS BEING IDENTICAL WITH A METAL WHOSE CHLORIDE IS CHOSEN FROM GROUP (2), WHEREBY AN ALLOY COATING OF THE METALS SELECTED FROM GROUP (3) IS FORMED ON THE SURFACE OF SAID REFRACTORY METAL ARTICLE.
 2. IN A PROCESS FOR BONDING A PLURALITY OF REFRACTORYBASE-METAL ARTICLES, SAID METAL BEING SELECTED FROM THE GROUP CONSISTING OF COLUMBIUM, COLIMBIUM-BASE ALLOYS IN WHICH THE COLUMBIUM CONTENT IS AT LEAST 50% BY WEIGHT, TANTALUM AND TANTALUM-BASE ALLOYS IN WHICH THE TANTALUM CONTENT IS AT LEAST 50% BY WEIGHT, THE STEPS COMPRISING (A) HEATING SAID REFRACTORY METAL ARTICLE FOR A PERIOD OF ABOUT FROM 1 TO 5 HOURS, AT A TEMPERATURE OF 800*C. TO 1000*C., IN A MOLTEN SALT BATH COMPRISING (1) A SALT SELECTED FROM THE GROUP CONSISTING OF ALKALI AND ALKALINE EARTH METAL HALIDES, (2) AT LEAST ONE METAL CHLORIDE SELECTED FROM THE GROUP CONSISTING OF TATANIUM DICHLORIDE AND CHROMIUM DICHLORIDE AND (3) AT LEAST TWO METALS SELECTED FROM THE GROUP CONSISTING OF TATANIUM, CHROMIUM, ALUMINUM, VANADIUM, AND ZICONIUM, AT LEAST ONE OF THESE METALS BEING IDENTICAL WITH A METAL WHOSE CHLORIDE IS CHOSEN FROM GROUP (2), WHEREBY AN ALLOY COATING OF THE METALS SELECTED FROM GROUP (3) IS FORMED ON THE SURFACE OF SAID REFRACTORY METAL ARTICLE, (B) PLACING A PLURALITY OF SAID COATED ARTICLES IN CONTACT WITH EACH OTHER, AND (C) SUBJECTING SAID CONTACTED ARTICLES TO AN ELEVATED TEMPERATURE AT LEAST SUFFICIENTLY HIGH TO EFFECT DIFFUSION BONDING THEREOF. 